The present invention relates to an image forming apparatus of the type using a revolver type developing device having a plurality of developing units therein.
Conventional image forming apparatuses include a full-color copier which exposes a photoconductive element or similar image carrier to color-separated light images to electrostatically form corresponding latent images, develops each of the latent images by a toner of complementary color, and transfers the resulting toner images to a single recording medium one above the other. A multicolor image forming apparatus is also conventional which sequentially exposes an image carrier to images to be reproduced in different colors, develops each of the resulting images by a developer of particular color to produce a corresponding toner image, and transfers such toner images to a single recording medium. This kind of image forming apparatuses need a plurality of developing units. However, a plurality of developing units constructed independently of each other and arranged around the image carrier would increase the overall size of the apparatus. A revolver type or rotary type developing device which is a recent achievement can eliminate this problem. The revolver type developing device, or simply revolver as referred to hereinafter, has a rotatable cylindrical casing located to face an image carrier, and a plurality of developing units disposed in the casing and supported in predetermined positions. The developing units are sequentially brought to a developing position to develop latent images by respective toners.
Regarding a copier, for example, monocolor documents, particularly black-and-white documents, are predominant over the other documents as far as daily office work is concerned. Hence, a toner of particular color, especially a black toner, is consumed in a particularly great amount. In the case of the revolver, the amount of toner which each developing unit can store is limited. In addition, it is difficult to connect large capacity toner containers to the outside of the revolver since the revolver bodily rotates. Moreover, supplementing the toner consumed in a particularly great amount from the outside of the device is inconvenient. In the light of this, Japanese Patent Laid-Open Publication No. 71981/1987, for example, proposes a copier which is usually loaded with a monocolor, e.g., black developing device for ordinary copy work and allows such a developing device to be replaced with a revolver type developing device for color copies, as needed. While this proposal pertains to two colors or so-called multicolor, e.g., red and blue, the above principle is also true with a greater number of colors or with a full-color developing device storing yellow, magenta and cyan toners.
Due to the spread of copiers and other image forming apparatuses, a current trend is toward an arrangement which allows the user to replace or otherwise manipulate individual units or an image forming device incorporating a plurality of units. Then, easy and safe replacement of the units and device and easy handling of substitute units and device are important considerations.
To load and unload the revolver with the units, the units may each be mounted on the front or the top of the apparatus body independently of the others, as disclosed in, for example, Japanese Patent Laid-Open Publication Nos. 208779/1985 and 127850/1987. However, the problem with this kind of scheme is that since the units are handled independently of each other, the replacement of the units for selectively dealing with black-and-white images or color images as stated above is not easy although the scheme may facilitate the replacement of the individual units and maintenance.
A developing device having a plurality of developing units constructed integrally with each other is taught in Japanese Patent Laid-Open Publication No. 78170/1988 by way of example. This kind of device allows all the developing units to be replaced by a single operation. However, since the units have to be pulled out at the front of the apparatus body, an extra mechanism for moving the units away from a photoconductive element before mounting or dismounting is needed. Further, to fully mount and dismount the units, slide rails and guides for facilitating the movement of the units in the longitudinal direction thereof are indispensable. As a result, there have to be provided a mechanism for promoting smooth mounting and dismounting work, and a rigid and reliable mechanism for protecting the units from damage due to a fall, preventing the toners from contaminating the surrounding and operator, and protecting the operator from injury. Such mechanisms are complicated and expensive, increasing the cost of the apparatus body. Moreover, the units cannot be pulled out at the front of the apparatus body unless the front side wall of the apparatus body is formed with an opening greater than the diameter of the units. The opening reduces the strength of the side wall and, therefore, makes it impossible to reduce the thickness of the side wall or requires some reinforcement. In addition, the opening has to be provided with a mechanism or member for positioning the units relative to the photoconductive element, further increasing the cost and complicating the mounting and dismounting work.
When the individual units or the revolver having any one of the above-described configurations is removed from the apparatus body, left at the outside of the apparatus body or transported, it is likely that the toner deposited on the casing and in the vicinity of the opening is scattered around to the outside. Further, a developer carrier is apt to appear via the opening of the casing to be damaged. Moreover, when the revolver is left at the outside of the apparatus or transported to another location, dust particles and other impurities are apt to enter the revolver via the opening of the casing. Such impurities would not only disturb images but also damage the constituent parts of the revolver.
As stated above, all the conventional schemes have defects regarding, among others, the replacement of the revolver. Specifically, with such schemes, it is difficult to replace or mount and dismount the units and position them in the event of mounting.